Operation - Analog Devices LTC2941 Datasheet

OPERATION

Overview
The LTC2941 is a battery gas gauge device designed for
use with single Li-Ion cells and other battery types with
a terminal voltage between 2.7V and 5.5V. A precision
coulomb counter integrates current through a sense
resistor between the battery's positive terminal and the
load or charger.
Coulomb Counter
Charge is the time integral of current. The LTC2941 mea-
sures battery current by monitoring the voltage developed
across a sense resistor and then integrates this infor-
mation to infer charge. The differential voltage between
+ –
SENSE and SENSE is applied to an auto-zeroed differ-
ential analog integrator to convert the measured current
to charge.
When the integrator output ramps to REFHI or REFLO
levels, switches S1, S2, S3 and S4 toggle to reverse the
ramp direction. By observing the condition of the switches
and the ramp direction, polarity is determined.
CHARGER LOAD
V
+
SENSE
R
SENSE
–
SENSE
I
BAT
+
GND
BATTERY
SUPPLY
S1
–
S2
S3
+
S4
Figure 3. Coulomb Counter Section of the LTC2941
For more information
A programmable prescaler is incremented or decre-
mented every time the integrator changes ramp direc-
tion. The prescaler effectively increases integration time
by a factor M programmable from 1 to 128. At each under
or overflow of the prescaler, the accumulated charge is
incremented or decremented one count. The value of
accumulated charge is read via the I
Power-Up Sequence
When V rises above a threshold of approximately
SENSE +
2.45V, the LTC2941 generates an internal power-on reset
(POR) signal and sets all registers to their default state.
In the default state, the coulomb counter is active. The
accumulated charge is set to mid-scale (7FFFh), the low
threshold registers are set to 0000h and all the high
threshold registers are set to FFFFh. The alert mode is
enabled and the coulomb counter prescaling factor M is
set to 128.
+
REFHI
CONTROL
LOGIC
–
+
POLARITY
DETECTION
–
REFLO
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LTC2941
2
C interface.
STATUS
CONTROL
REGISTERS
PRE-
2
I C/
SCALER
SMBus
M
ACR
THRESHOLDS
AL/CC
SCL
SDA
2941 F03
Rev. C
7